In recent years, as the next-generation solid state nonvolatile memory capable of carrying out operation with high speed read/write, high capacity, and low power consumption, people have paid much attention to magnetic random access memory (hereinafter to be referred to as MRAM) exploiting the magnetoresistive effect of a ferromagnetic material.
In particular, magnetoresistive elements having a ferromagnetic tunnel junction have attracted much attention as they have been found to demonstrate a significant ability to change magnetoresistivity. The ferromagnetic tunnel junction has a 3-layer laminated structure including a memory layer where the magnetization direction is changeable, an insulator layer, and a fixing layer that maintains a prescribed or intended magnetization direction in the memory layer.
A magnetoresistive element having a ferromagnetic tunnel junction is also called a magnetic tunnel junction (MTJ). People have proposed using a spin injection write system using spin momentum transfer (SMT) as the memory write system for MTJ's.
According to the spin injection write system, as a spin polarization current flows in the magnetoresistive element, the magnetization direction of the element is inverted. As a consequence, the volume of the magnetic layer needed to form the memory layer decreases and thus the current to inject spin polarization electrons decreases. Consequently, it is expected that a spin injection based writing system may have both very small memory cell and unit size and low current requirements for switching.
The ferromagnetic material that forms the magnetoresistive element may be the so-called vertical magnetization film, a film having a simple axis of magnetization.
When crystal magnetic anisotropy is exploited in the constitution of the vertical magnetization type, shape magnetic anisotropy is not exploited, so that the size of the element can be smaller than the in-plane magnetization type. Also, as the dispersion in the easy axis of magnetization is small, by adopting a material with a high crystal magnetic anisotropy, it is possible to realize both finer structure and lower current while maintaining a high resistance to thermal disturbance.
Usually, the magnetostatic stray field generated of the reference layer in the MTJ element where the memory layer and the reference layer are of the vertical magnetization type is higher than that of the MTJ element of the in-plane magnetization type.
Also, a memory layer with a coercive force lower than that of the reference layer is significantly influenced by the leak magnetic field of the reference layer. More specifically, under the influence of the magnetostatic stray field of the reference layer, thermal stability degrades, and switching current value and its range increase. This is undesirable.